Shaman Ring

The Shaman Ring (called Sherman Ring in Aria of Sorrow) is an accessory in the Castlevania series which causes the protagonist to gain increased experience throughout their quests, be it from gaining it via just walking, or by increasing it through killing monsters.

Note that in Portrait of Ruin, equipping Shaman Rings on both of Jonathan or Charlotte's accessory slots will increase all EXP gained by around 56.25%, instead of only 50%, indicating that the boost is multiplicative.